Skybox rendering offset fix for third person/alt view camera

Fixes the issue reported in this thread: https://mb.srb2.org/showthread.php?t=41729

I dunno if this will negatively affect any existing skyboxes in SRB2's own levels, that said. I tried out THZ2 and CEZ1 with this fix at least but I forgot to compare them with how they are in 2.1.15 so *shrugs*